1. Field of the Invention
The present invention relates to a synchronization establishing method of a mobile station in a mobile communication system, and more particularly to a synchronization establishing method of a mobile station in a mobile communication system for carrying out peak detection by averaging the synchronization channel in phase in complex numbers.
2. Description of the Related Art
In a W-CDMA (Wide-band Code Division Multiple Access) scheme, a radio access scheme of a third generation mobile communication system, a synchronization channel (SCH) is multiplexed onto a downlink signal transmitted from a base station to a mobile station to enable the mobile station to carry out synchronization establishment by detecting the downlink signal (see, 3GPP Technical Specification 25.211).
Usually, the SCH is transmitted to the mobile station at a known frequency, in known time slots, in a known code and symbol pattern. To carry out synchronization establishment, that is, the timing detection of the SCH, the mobile station performs correlation detection at the frequency, in the time slots and in the code and symbol pattern, thereby determining the most likely timing as the timing of the SCH.
FIG. 1 is a diagram showing a commonly used power averaging method. In FIG. 1, the reference numeral 201 designates the output from a correlator of a mobile station, that is, correlation values of a synchronization channel transmitted from a base station for initial synchronization. The reference numeral 203 designates an equipment to powerize for calculating a square sum of the correlation values of the synchronization channel; and 205 designates a peak detector for detecting a peak of the correlation values in each period of the synchronization channel.
The SCH is repeatedly transmitted at predetermined intervals (at intervals that will give peak values of the output). It is assumed in the following description that the interval is one slot. Normally, to reduce the effect of noise, interference and received power fluctuations, the mobile station averages the correlation values of the SCH over a plurality of slots. Conventionally, the mobile station carries out the averaging after powerizing the output of the correlator slot by slot (power average).
Thus, commonly used power averaging method carries out averaging of the output of the correlator of the mobile station using an equipment to powerize for converting correlation values of the synchronization channel transmitted from the base station into power dimension for establishing the initial synchronization, and a peak detector for detecting the peak of the correlation values in each period of the synchronization channel.
FIG. 2 shows a structure of the SCH. In one frequency, synchronization channels 103 are assigned periodically to a channel set 101 and are transmitted to the mobile station at known frequency and time slots in known code and symbol pattern. The pilot symbols are also used for measuring received signal power. The reference numeral 111 designates a graph representing correlation outputs of the synchronization channel versus time (timings).
The SCH is transmitted to the mobile station at known frequency and time slots in known code and symbol pattern. In the example of FIG. 2, the SCH is transmitted at every one slot interval. The peak detector 205 of the mobile station detects the timings (peaks) 113 at which the correlation values take a maximum value from the correlation values by the correlator at all possible timings. In the conventional power averaging, the averaging is carried out after powerizing the output of the correlators slot by slot as shown in FIG. 1.
Propagation paths of the mobile communication have interference or noise, and the signal-to-noise (S/N) ratio of the SCH for the mobile station is usually very small. Since the SCH is one of control channels that do not transmit user information, it is preferable that the transmission power and transmission duration of the SCH be as small as possible from the viewpoint of the system capacity. To achieve synchronization establishment at high accuracy in such severe conditions, the averaging period must be lengthened. However, an increasing averaging period will lengthen the time required to carry out the synchronization establishment. As a result, it will provide a problem of increasing the power consumption of the mobile station, and lengthening the time taken by cell switching control (handover) involved in the movement of the mobile station across cells during communication.
In addition, mobile communication systems sometimes employ time switched transmit diversity (TSTD) that installs two antennas on a base station and transmits the signal alternately in a prescribed pattern at every predetermined interval to reduce the effect of received level fluctuations due to fading. For example, to apply the TSTD to the SCH, the base station can transmit the SCH alternately in a prescribed pattern at every slot interval from the two antennas (see, 3GPP Technical Specification 25.211). In this case, since the phases of the signals transmitted from the two antennas usually differ from each other, it is difficult to average them in phase.